1. Field of the Invention
The present invention relates to a rotor for an induction motor and, more particularly to the rotor that reduces loss of the magnetomotive force of the induction motor for thus improving a start-up efficiency of the induction motor, and that is reduced in weight thereof for greatly reducing the manufacturing cost therefor.
2. Description of the Prior Art
In general, as shown in FIG. 1, an induction motor has a core formed of a plurality of stacked circular rotor cores, each being made of a Si-steel plate, and a stator having a primary winding from which a magnetic field is generated, wherein the rotor is positioned within an effective range of the generated magnetic field. Thus, the location of the rotor as a secondary conductor causes the magnetic flux to be formed over the bulk of the rotor due to a circular shape of the rotor cores.
A rotor 1a used in a conventional induction motor as shown in FIG. 14A is configured such that in the central portion of a plurality of stacked rotor cores 2a, is formed a shaft inserting hole 3a for inserting a motor shaft 11 (FIG. 1), and an outer portion of the shaft inserting hole 3a is provided with many blowholes 6a for an air flow.
Another induction motor conventionally provided includes, as shown in FIG. 14B, a plurality of stacked rotor cores 2b having a shaft inserting hole 3b at a central portion thereof prepared for inserting a motor shaft, and a plurality of blowholes 6b at an outer portion thereof for an air flow. Furthermore self-stacking recesses 8 used for automatically stacking rotor cores 2a are formed outside of the blowholes 6b of the rotor core 2b.
However, for each of rotors 1a, 1b in two examples stated above, the magnetic flux is wholly distributed throughout the circular rotor cores 2a, 2b, resulting in the production of relatively longer paths among the magnetic flux paths. The more long paths for the magnetic flux are, the higher increased magnetic reluctances are generated.
Accordingly, there is a problem in that higher magnetic reluctance acts as a cause of the considerable loss of magnetomotive force which consequently reduces a start-up efficiency of the motor.
Furthermore, there is another problem in that heavier rotors 1a and 1b result in increased no-load torque and inertia force, caused by their heavy weight, and lead to an increase in the manufacturing cost.